Frequency modulation



March 1931- G. L. USSELMAN 1,794,932

FREQUENCY MODULATION Filed Sept. 1, 1927 Hlllllhll FIAT! (URREI/I' P6? 72/85 INVENTOR GEORGE L. USSELMAN A TORNEY GEORGE L. USSELMAN, OF ROCKY roman, NEW YORK,

TION OF AMERICA, A CORPORATIONOF DELAWARE FREQUENCY uonorarron Application filed September This invention relates to signalling by frequency modulation, and more particularly to the analysis and detection of received frequency modulated signals.

Frequency modulation, in the ideal case, should be accompanied by no amplitude mod- .ulation.- However, in actual practice a re ceived frequency modulated wave often does show amplitude variation, for various reasons, such as fading, or fluctuations in the power input to the transmitter, which may occur at a disturbing frequency. The usual type of analyzer circuit consists of a resonant circuit tuned to a frequency lying to one side of the operating frequency range, and its purpose is to change frequency modulation to amplitude modulation. It is obvious that such a circuit will respond to amplitude modulation and therefore the output will be impure. It is an object of my invention to provide an analyzer circuit which will change frequency modulation to amplitude modulation but which will'not respond to amplitude modulations of the energy supplied to it.

It is desirable that the analysis be made according to a straight line resonance characteristic in order that the amplitude modulation be a true reproduction 'of the original frequency modulation, to avoid the production of harmonics, and to prevent intermodulation, of multiplex signals. It is also dcsirr able that if the resonance characteristic be curved its curvature be symmetrical in order that the positive and negative halves of the alternating current component of the amplitude modulated energy be identical. For exactly similar reasons it is desirable that the subsequent rectification or detection of the amplitude modulated energy be according to asymmetrical characteristic.

It is proposed to use for multiplex signalling a carrier which is frequency modulated in accordance with a complex low frequency wave made up of a number of lower frequencies each of which represents a signalling channel. The selection of suitable channel frequencies is, greatly restricted, ordinarily, due to the necessity of avoiding the selection of frequencies which are mutually related as harmonics, owing to the fact that harmonic 1, 1927. Serial No. 216,873.

frequencies may be produced naturally even when not transmitted. If natural harmonics are suppressed then harmonics may be used as channel frequencies and the available choice thereby be greatly widened. Accordingly, it is a further object of my invention to provide a balanced circuit which will suppress undesired harmonics.

The above described objects are attained by supplying the frequency modulated energy cophasially to each of two circuits having intersectin resonance curves with maxima at frequencies lying either side of the operating range of frequency, applying the potential fluctuations from th the control electrodes of two detector tubes in phase opposition, and combining the outputs from the detector tubes 1n phase opposition. Frequency changes cause one of the two circuits to become more strongly energized than the other, and as the frequency swings to and fro an alternating current component is found in the series connection between the anodes of the two tubes. On the other hand, amplitude variations, while causing similar variations in the outputs ofweach of the analyzer circuits, do not affect the resultant output because they are combined in opposition. Because each half of .theresonance characteristic consists of a similar portion of each of two resonance curves the resonance characteristic and because the effective or combined characteristic is the resultant of two differently curved characteristics the resultant'is more nearly straight, and a wider frequency band may be utilized if desired. In an exactly similar way the resultant efiective rectification characteristic is made more symmetrical, and by reason of an inherent property of symmetrical circuits numerous harmonics are suppressed.

The analyzer, as-so far described, requires the use of two analyzer circuits instead of one, which might complicate the operation of tuning the receiver. It is a further object of my invention to provide for uni-control of the two circuits, and this is most simply done by employing arrangements quite similar to those which have been developed in connec- ASSIGNOR 'J!O RADIO CORPORA- is symmetrical,

tion with uni-control heterodyne receivers. In simple form this may consist of a straight line tuning condenser in each of the two analyzer circuits, with means to vary them simultaneously, and additional means to adjust their frequency separation.

Owing to unavoidable variations in the I mean carrier frequency transmitted from the mean frequency of the energy supplied to it,

and this I do by employing in combination with the anaylzer as so far described a relay which is responsive to the difference in the average values of a portion of the outputs 1 invention.

from the higher frequency and lower frequency circuits to adjust their tuning so as to tend to lessen the difference. In this way the tuning of the two circuits is simultaneously changed so that the frequencies to which they are resonant will always straddle the mean frequency of the energy supplied to the analyzer circuits. The invention is described more in detail 1n connection with the accompanying drawings in which Figure 1 is a wiring diagram and schematic representation of one form of my invention; that because the on vature of the combined halves of the worki g portions of the reso- Figure 2 is-a resonance curve for the analyzer circuits; and Figure 3 represents another form of my Referring to Figure 1 frequency modulated energy is supplied by means of the conductors 2 to the resonant analyzer circuits 4 and-6, by way of coupling transformers 8 and 10. Potentials of opposite phase are supplied from the circuits 4 and 6 to the control electrodes of the detector tubes 12 and 14. The anodesof the tubes are connected in series by an output transformer 16, which is bypassed for radio frequencies by a condenser 18. As so far described this circuit is slightly similar to many symmetrical circuits now known,

dependent input circuits 4 and 6 which are di erently tuned.

The operation of this circuit may be more readilyexplained in connection with'Figure 2, in which 'curVeAO is a resonance curve for the circuit 4, while the curve 1s a resonance curve. for the circui 6. These curves have maxima for frequencies F and F 'respectively which lie either side of the mean frequency F The operatin frequency range may be representedbyasha ed area, as shown. It will be noted that't-he. resonance curves intersect, and this is a requirement for suc- 7 cessful operati 11, but in view of the'fact that relatively smal employed when frequency modulating, this output.

frequency ranges are to be requirement will be fulfilled automatically in ractice.

ssume that the instantaneous frequency is F In such case the circuits 4 and 6 will be excited to equal potentials, and the outputs in the primary of the transformer 16 This will be will neutralize one another. the zero potential point of the low frequency As the frequency swings toward the lower frequency F the lower frequency circuit 4 will be more strongly energized, and

the higher frequency circuit 6 more weakly,

energized,'in consequence of which there will be a current flow in the transformer 16. As

the frequency again reaches the mean frequenc F alternating current flow in the trans ormer 16 again will pass through zero. When the frequency increases towards F 2 the hi her frequency circuit 6 will be more strong y energized, and the lower frequency circuit 4 more weakly energized, and there will be a resultant current flow through the transformer 16 in the opposite direction.-

In'this way there is response to frequency modulation. But because the 'inputto the circuits 4 and 6 is in one phase relation, whereas the outputs from the circuits 4 and 6 are in opposite phase relation, it follows that amplitude variation in the input are opposed in the combined output, and therefore are neutralized. f

It can be seen from an inspection of Figure nance curves are curv'ed in opposite sense that the resultant curve will be more nearly circuits will be obtained, for example, a symmetricalresultant characteristic, the elimination' of harmonics, and the withdrawal from the direct current source of practically constant current, thereby keeping the applied but it is importantly different in the use of in- Potential more nearly uniform- The tuning condensers inc'ircuits 4 and 6 may be arranged to be vrotated by a single control, and in such case their frequency sepgeared to the condensers by'drive chains 22; The position ,of the condensers relative to their respective chain wheels may be varied by adjusting the location of lock screws 24 in. arcuate slots 26.

concomitant with the use of. balanced tube For keeping the analyzer circuits auto- I maticall'y in tune the steadyv components of the anode currents are passed through opposed relay magnets 30, which are by-passed.

received high frequency,

by large blocking condensers 32. These magnets influence the armature 34 which is arranged to close a current supply from a line 36 to either side of the reversing relay 38 of a motor 40. A worm gear 42, on the shaft of the motor 40, engages with a worm wheel 20, and thereby serves to vary the tuning of the analyzer circuits;

Attention is now directed to Figure 3 which is similar to F i ure 1 except for some alternative features w ich are next described. The relay magnets 50 are in this case intended to have'such high inductance that only the direct current component of the anode currents will pass therethrough. There are thus provided three paths in parallel, which according to their respective impedances, serve to separate the desired output alternating energy, the high frequency energy, and the steady anode current. The arrangement shown in Figure 1 is preferable only in this respect, that it is cheaper to provide a blocking condenser, such as those indicated at 32, than it is to provide relay magnets which will efficiently prevent loss of the low frequency alternating component.

Figure 3 also difi'ers in that the detecting action of the tubes 12 and 14 is obtained by grid bias, ratherthan by the use of a condenser and grid leak. The choice of bias means is of little consequence except that it must be remembered that the average anode current varies in opposite sense, that is, in the case of rectification by grid condenser the anode current decreases for increased al- .ternatin energy, whereas in the case of negative gri bias the anode current increases for increased alternating energy, and therefore the connection to the automatic tuning control must be reversed.

Figure 3 further differs in that a different means for controlling the frequency separation of the resonantcircuits is indicated. The

' underlying idea is to move the rotors of the condensers together, and to move one of the stators in order to adjust the frequency separation. This has been indicated by a wheel 52 which coacts with the stator, when rotated by a dial 54 and so permits adjustment of the relative stator position. This arrangement is especially adapted for putting the condensers and the wheel 20 all on a single shaft, mechanically.

It is to be understood that the input tothe analyzer circuit may be energy of intermediate frequency obtained by heterodyning the and that the output from the detector may be separated into various channels in the case of multiplex signalling. The detectors may be any suitable device of asymmetric characteristic.

I claim:

1. The method of discriminating between amplitude modulation and frequency modulation when analyzing frequency modulated energy which includes supplying the energy in a desired phaserelation to each of two circuits tuned to frequencies lying either side of the operating range of frequency, and combining the outputs from the two circuits in an opposite phase relation.

2. The method of obtaining a symmetrical and substantially straight resonance characteristic for analyzing frequency modulated energy which includes supplying the energy in one phase relation to each of two circuits having intersecting resonance curves with maxima 1 ing either side of the operating range of f iequency, and combining the outputs from the two circuits in an opposite phase relation.

3. The method of obtaining symmetrical and substantially straight resonance and symmetrical detection characteristics for analyzing and rectifying frequency modulated energy which includes supplying the energy cophasially to each oftwo circuits having intersecting resonance curves with maxima lying either side of the operating range of frequency, detectin the outputs from the two circuits, and com ining the de tected outputs in phase opposition.

4. The method of eliminatin the occurrence of undesired harmonics w en rectifying analyzed frequency modulated energy which includes detecting a. portion of the analyzed energy, detecting another portion of the analyzed ener and combining the detected outputs in ase opposition.

5. The method 0 analyzing and rectifying frequency modulated energy which in cludes sn 1 in the ener in one base relation t dabh f two circi l i ts having ihtersecting resonance curves with maxiina lying either side of the operating range of freuency, separately etecting the potential uctuations in each of the two circuits in an opposite phase relation, and combining the detected outputs in opposite phase relation.

6; The method of tuning a balanced analyzer for frequency modulated r'p'nais comprising circuits tuned to highe" 3.341 lower frequencies which includes comparing the average values of portions of the outputs of the higher freq frequency circuit, and varyin the tuning of the analyzer circuits in that irection which reduces the difference between the compared values. a

7 The method of tuning a balanced analyzer and detector for frequency modulated signals com rising circuits tuned to higher and lower requencies and detectors which includes comparing the magnitudes of the steady components of the higher frequency and the lower frequency detector outputs, and adjusting the tuning of the analyzer circuits in that direction which makes the'steady components equal. 8. The method of keeping a balanced anauency circuit and the lower .1

matically in tune whichincludes opposing the steady components of the higher frequency and the lower frequency detector outputs, and utilizing the difference in. the opposed components to change the tuning of the analyzer circuits in the direction which reduces the difference.

9. An analyzer for energy which is frequency modulated in accordance with a desired signal comprising a resonant circuit tuned to a frequency lying at one side of the operating range of frequency, another resonant circuit tuned to a frequency lying at the other side of the operating range of fre quency, means to supply the frequency modulated energy to the two circuits in a desired phase relation, and means for combining the outputs from the two circuits in an opposite phase relation to obtain energy modulated in amplitude in accordance with the desired signal.

10. quency modulated in accordance with a desired signal comprising two resonant circuits having intersecting resonance curves with maxima lying either side of the operating range of frequency, means to supply the frequency modulated energy to the two circuits -in .one phase relation, and means for combining the outputs from the two circuits in an opposite phase relation toobtain energy modulated in amplitude in accordance with the desired signal.

11. An analyzer and detector for frequency modulated signals comprising two resonant circuits having intersecting resonance curves with maxima lying either side of the operating range of frequency, two rectifying means, means to supply the frequency modulated energy to the circuits in one phase relation, means toapply energy of an opposite phase relation from the cirruits to the rectifying means, and means to combine the outputs of the rectifying means preparatory to translation of the signal.

12. An analyzer and detector for frequency modulated signals comprising two resonant circuits having intersecting resonance curves with maxima lying either side of the operating range of frequency, two detector tubes having control electrodes, means to supply the frequency modulated energy 'to the circuits cophasially, means to apply potential fluctuations fromthe circuits to the control electrodes in phase opposition, and means to combine the tube outputs in phase opposition preparatory to translation of the signal.

13.An automatically tuned balanced analyzer comprising two'resonant circuits having intersecting resonance curves with ma nmalying either side of the operating range An analyzer for energy which is freof frequency, variable reactances for tuning said circuits, means to supply energy to the circuits in one phase relation, means to combine a portion of their outputs in an oppo- 14. An automatically tuned balanced an-' alyzer and detector comprising two resonant circuits having intersecting'resonance curves with maxima lying either side of the operating range of frequency, variable reactances for tuning said circuits, a plurality of detectors, means to supply energy to the circuits in one phase relation, means to feed the detectors from the circuits inan opposite phase relation, means to combine the alternating components of'the detector outputs likewise in opposite phase relation, and means responsive to a difference in the steady components of the detector outputs for automatically changing the magnitude of the variable reactances to lessen the difference.

15. An analyzerfor frequency modulated energy comprising two resonant circuits having intersecting resonance curves with maXima lying either-side of the operating range of frequency, means to supply the frequency modulated energy to the twocircuits in one phase relation, means for combining the outputsfrom the two circults in an opposite phase relation, a straight line frequency condenser in each of the resonant circuits, means to vary them simultaneously, and "means to I adjust their frequency separation.

16. An analyzer and detector for frequency modulated signals comprising two resonantcircuits havin intersecting resonance curves with maxima Tying either side of the operating'range of frequency, two rectifying means,

means to supply the frequency modulated 17 An analyzer and detector for frequency modulated signals comprising two resonant circuits having intersecting resonance curves with maxima lying either sideof the operating range of frequency, two rectifying means, means to supply the frequency modulated energy. "to the circuits in one phase relation,

means to apply energy of an opposite phase relation from the circuits to the rectifying L means, means to combine the outputs of the rectifying means, a straight line frequency condenser in each of the resonant circuits, a reversible motor for varying the condensers simultaneously, means to adjust their frequency separation, and a relay responsive to thedifi'erence in the steady components of the detector outputs for operating the motor .in that direction which lessens the difference.

18. An analyzer and detector for frequency modulated signals comprising two resonant circuits having intersecting resonance curves with maxima lying either side ofthe operating range of frequency, two detector tubes having control electrodes, means to supply the frequency modulated energy to the circuits cophasially, means toapply potential fluctuations from the circuits to the control electrodes in phase opposition, means to combine the tube outputs in phase opposition, a straight line frequency condenser ,1n each of the resonant circuits, a reversible motor for varying the condensers simultaneously,

means to adjust their frequency separation, and a relay responsive to the difference in the steady components of the detector outputs for operating the motor in that direction which lessens the difference.

' GEORGE L. USSELMAN. 

